I was doubt in Irene, for some reasons:
1) When a gun is in the possession of someone,I excluded from their suspected persons (As happens in many Detective Conan episodes haha).
2) Mycroft didn't know when Irene will come back.
3) She delayed for 10 minutes.
4) Though Mycroft hear her flush the toilet, It may be sound recorder.
~But my inference was all wrong ): !